30

R app. P.-v. Réun. Cons. int. Explor. M er, 168: 30-34. Janv ier 1975.

ON T H E USE OF BRA KE H O R S E PO W ER AS A PA R A M E T E R F O R F IS H IN G PO W ER

E .J. d e B o e r

Technical Fisheries Research Departm ent, IJm uiden, The Netherlands

IN T R O D U C T IO N

For some considerable time the fishing power of fishing vessels has been related to various technical parameters of such vessels. One of the parameters applied to fishing methods using a towed gear is the power of the engine. The power of diesel engines is indicated by the Brake Horse Power (BHP; Pb *)1- For definitions of the ratings of marine diesel engines see Appendix A.

The word “brake” indicates the method of meas­uring the power. The determination of the BHP is per­formed at the testbed by means of a waterbrake. The definition of BHP is the maximum power the engine can continuously deliver when running at nominal revolutions.

For vessels having a steam engine as the main pro­pulsion system, the power is expressed as Indicated Horse Power (IH P; Pi*). The word “ indicated” denotes the power determined by the mean effective pressure in the cylinders calculated from an indicator diagram. The power a steam engine can deliver to the propeller is far less due to internal losses in the engine.

In the following, attention will be confined to pro­pulsion systems using a diesel engine.

T H E D E L IV E R ED PO W E R A T T H E PR O PE L L E R

For the propulsion of a vessel the power available for driving the propeller is of importance. This power is indicated by “ delivered power at the propeller” (DHP; Pd*)- The delivered power at the propeller is the power measured in the propeller shaft, just in front of the hub. In Appendix B a summary of power definitions of propulsion systems is given.

The delivered power at the propeller is the power the propulsion system delivers under the momentary operating conditions decreased by:

(a) the power consumption of machinery driven by the main engine, gearbox (power-take-off) and propeller shaft (generators, pumps, etc.), and

1 All symbols m arked with an asterisk (*) are as adopted by the International Towing T ank Conference.

(b) the losses in shaft bearings, gearbox, thrust block, etc.

The losses in the propeller shaft are approximately 3% of the BHP. The efficiency of a marine gearbox of good quality with single reduction gearing is 97%.

Because of the increase in the number of energy consumers on fishing vessels due to continuing mechan­ization and automation, the power needed for general service augments considerably. O n Dutch fishing ves­sels the general service generator is driven by the main engine. Nowadays 50-80 kW general service gener­ators are normal for 600-800 BHP trawlers.

Fishing vessels having a considerable freezing capa­city require a lot of power for the freezing plant. In most cases this power is also delivered by the main engine. A 1700 BHP freezer trawler, for example, under fishing conditions and simultaneously freezing at 70% capacity, has approximately 1300 DH P avail­able for propulsion.

Thus, the use of the BHP of the engine as a para­meter for fishing power is not to be recommended. A far better parameter is the DHP under fishing con­ditions.

T H E PR O PE L L E R

When the power available for driving the propeller (Pd) and the rotational speed of the propeller (n) are known, the torque (Q_) can be calculated. The pro­peller converts this torque (£)) into a thrust ( T ) . The thrust ( T ) gives the vessel and its gear a speed (V) through the water. At this speed the thrust will be equal to the sum of the resistance forces (R t )■

The resistance forces during fishing operations are the wind and water resistance of the vessel and that of the gear, the latter being far bigger than the former.

The best parameter for the propulsive performance during fishing operations is the effective power (EHP: Pe*) defined by the product R t - V .

The ratio between the effective power (Pe) and the

Brake horse power as fishing power parameter 31

delivered power at the propeller (Pd) is the Quasi Propulsive Coefficient (QPC; r]D*)-

Vo = Pe /Pd = (Rt ‘ V)l(2n • Q,- n)

When designing a propeller the aim is to make r\D as high as possible for the selected operating condi­tion (s).

The rjD can be subdivided into the following parts: (1) the open water efficiency of the propeller (fjo), (2) the relative rotative efficiency (rjR), and (3) the hull efficiency (rjij).

The efficiency of the propeller in open water is the efficiency under tank conditions. This means the propeller is not working behind a vessel, and there is no influence of vessel, rudder, etc. on the performance of the propeller. The open water efficiency is the most important of these three efficiencies and varies con­siderably depending on the operating conditions of the propeller.

The relative rotative efficiency for single screw ves­sels is approximately 1-05.

The hull efficiency depends on the wake fraction (w) and the thrust deduction coefficient (£). The for­mer is constant for a given vessel (w = — 0-05 + 0-5 Cb ; where Cb is the block coefficient) while the latter varies with the speed of the vessel (t = 0-05 + 0-15 V; where V is the speed in knots).

TYPES O F PR O PELLER S

One can classify propellers according to possible pitch settings, namely fixed pitch propellers (fpp), controllable pitch propellers (cpp) and adjustable pitch propellers (app).

The fixed pitch propeller is limited in its working performance by the fixed pitch setting. The adjust­ment of the desired thrust is performed by the varia­tion of the rotational speed of the propeller.

The controllable pitch propeller allows the best pitch setting to be selected for each working condition. The adjustment of the thrust is performed by the variation of the rotational speed and/or the pitch.

The adjustable pitch propeller has, for example, two pitch settings, one for towing and one for free running.

In order to illustrate the factors influencing the per­formance of a propeller some examples, referring to fixed pitch propellers, are given below.

Design speed

Design speed is the speed of the vessel when the maximum power available for driving the propeller is fully absorbed. If the fishing speed is less than the design speed the propeller, at full rotational speed, needs a bigger torque (Q_) than for the design condi-

2A0 rpm

800CL

x 700

œ 600LU

- 500

750

—iUJo A00ci?

fishingspeed

design speed A-0 knots10,000

T O -t)ID 3crxI-

5,000pull 7800 kgfI

UJ<_>

SPEED (knots)

Figure 12. Propulsive performance of an 800 H P traw ler with a design speed o f 4-0 knots.

tion. However, the maximum torque is delivered at the design speed and due to the constant torque char­acteristic of the diesel engine, the main engine will decrease in revolutions to a value where the absorbed torque of the propeller is equal to the maximum torque of the main engine. The result is that the delivered power at the propeller will decrease. The decrease of delivered power becomes greater if the difference be­tween fishing speed and design speed increases. The power of the main engine is approximately propor­tional to the number of revolutions.

Figures 12, 13 and 14 show the propulsive per­formance of an 800 BHP trawler, the propeller being designed for 4-0, 9-0 and 12-3 knots respectively. The delivered power at the propeller is 750 DHP. The values of the maximum available pull at a speed of

32 E. J . de Boer

Lü 240 r

220218 rpm£ 200 ^

i . 180 -k_

800

700

600 675 DHP

o 400 riP

fishingspeed

design speed 9-0 knots10,000

CTT(1-t)

5,000i—

7100 kgf

cc

SPEED (knots)

Figure 13. Propulsive perform ance of an 800 H P traw ler with a design speed of 9’0 knots.

4-0 knots are 7800 kgf, 7100 kgf and 6400 kgf respec­tively.

The diagrams show that design speed has a great influence on the propeller performance when towing (and free running). The choice of the design speed depends mainly on the sailing distance to the fishing grounds. For the Dutch double-rig beam trawlers the maximum distance is 200 nautical miles. Because the influence of the free running speed on the number of useful fishing hours per trip is small and the resistance of this type of vessel increases tremendously for speeds exceeding 10 knots, the design speed of the propeller is, in general, 4 knots. The Dutch wet fish and herring trawlers, many of which have been converted to beam trawling, have propellers with a design speed of 9 knots (compromise propeller) or more (free running

240LÜ—I

UJ 220 -

S 200 -

E 180 =-CL

160 L

198 rpm

i800

700

600 616 DHP>

Q_

design speed 12-3 knotsfishimspeec10,000

T (1-t)na.jE 5,000

pull 6400 kgfUJo

SPEED (knots)

Figure 14. Propulsive performance of an 800 H P traw ler with a design speed of 12-3 knots.

propeller). Comparing the fishing power of these ves­sels with specific beam trawlers having the same engine output and without taking into account the design speed of the propellers gives a wrong picture of the actual fishing power ratio.

Rotational speed of the propellerThe rotational speed can be predicted by the choice

of the main engine. When using main engines with an output of less than approximately 1200 BHP, and nominal speed of less than 400 rpm, it is unusual to reduce the rotational speed of the propeller by means of a gearbox. In this case the rotational speed of the propeller is fixed. The optimum diameter Dopt (and the pitch P) is calculated for this rotational speed; the

Brake horse power as fishing power parameter 33

optimum diameter being the diameter for optimum efficiency.

One of the limiting factors for the rotational speed of the propeller is the blade tip speed. In general this speed should not exceed a value of 36 m/sec in order to avoid cavitation (cavitation damage and decrease in efficiency).

The influence of the rotational speed on the fishing power can be illustrated by the following example. For a 1300 DHP Dutch stern trawler the propeller has a rotational speed of 380 rpm and the propeller dia­meter (D) is 2060 mm. The thrust T (1 — t) at 4 knots is 11000 kgf. For the same installation, but with a gearbox reducing the rotational speed of the propeller to 250 rpm, Dopt = 2600 mm. The thrust T (1 - t) at 4 knots increases to 12700 kgf (i.e. by 15-4%).

It will be clear that the rotational speed of the propeller strongly influences the fishing power and that it has to be taken into account when comparing fishing powers.

Diameter of the propeller

According to the axial momentum theory the high­est efficiency is obtained at the largest possible dia­meter at optimum rotational speed. In general the tendency is therefore to select a propeller with as large a diameter as possible and calculate the optimum rota­tional speed nopt. This optimum rotational speed determines for a certain type of main engine the re­duction ratio of the gearbox. However, the propeller diameter is limited by the draught aft (T a). In order to prevent the propeller coming above the water sur­face due to the vessels’ pitching (decrease of propulsive performance and increase of cavitation), the diameter has to be D < 0-7 T a . The draught aft ( T a) depends mainly on the dimensions and the construction of the vessel. A limitation to the draught aft can be the re­stricted depth of the entrance to the harbour, as is the case for some harbours in the Netherlands. In such cases an increase in engine power above a certain limit gives only a relatively small increase in propul­sive power and is not to be recommended. Never­theless, in the Dutch fishing fleet, several vessels have propellers with too small a diameter. Another reason for too small a propeller diameter in relation to the power of the main engine is the replacement of the main engine by a more powerful one without any in­crease in the diameter of the propeller due to the dimensions of its aperture.

An example of the influence of the propeller dia­meter is the following comparison of two 705 BHP beam trawlers. One has a restricted diameter of 1900 mm (nopt = 307 rpm), and delivers at 4 knots a thrust T (1 — t) of 7300 kgf, while the other one has

cr240 1- 1

1220 -

111

200 — ©— !------------------------------------------

180 -200rprp

ii

800

750DHP700

£ 600 UJ

Ë 500

400

fishingspeed

design speed 4-0 knots ducted propeller

10,000

tn2Xh-

9200 kgf5,000

iLÜ

§

ILOUJcr

SPEED (knots)

Figure 15. Propulsive performance of an 800 H P trawler fitted with a ducted propeller and having a design speed of 4-0 knots.

a propeller diameter of 2050 mm (nopt = 248 rpm) resulting in a thrust T (1 - t) of 7860 kgf at 4 knots.

Ducted propellers

The ducted propeller is a propulsion system that, for various ranges of operating conditions, meets the main requirements for ship propulsion systems. In the Dutch fishing fleet the number of ducted propellers is increasing continuously.

Figure 15 shows the propulsive performance of an 800 BHP trawler with a ducted propeller. Due to the nozzle, the propeller diameter has to be decreased; nevertheless, at 4 knots a pull of 9200 kgf is available.

The nozzle has a great influence on the performance

34 E. J . de Boer

of the propeller. Several Dutch beam trawlers claim an increase in pull of between 12 to 18% under fishing conditions.

C O N C LU SIO N S AND R E C O M M E N D A T IO N S

1. I t is recommended that the “delivered power at the propeller” under fishing operations be used as a fishing power parameter instead of the BHP of the main engine.

2. In the case of a ducted propeller the value of the delivered power at the propeller has to be multi­plied by a correction factor because of its higher efficiency compared with an unducted propeller.

3. In order to correct for the differences in propeller efficiency a factor depending on the rotational speed of the propeller has to be determined.

A PPE N D IX A

RATINGS O F M A R IN E D IESEL ENGINES

Maximum rating. The maximum output the engine is capable of developing for periods not exceeding five minutes duration. The engine is stripped of all acces­sories. This output is obtainable under standard con­ditions of temperature and altitude on dynamometer test only.

Intermittent rating. The maximum output permissible for applications which require variable speed and variable load. The average load factor has to be in the region of 50-70%. The fuel pump is set to limit the output of the engine to this rating. Applications of this rating are fork trucks, cranes, belt conveyors, roadrollers, etc.

Continuous rating. The output the engine can develop continuously at nominal speed under standard con­ditions of ambient temperature and altitude. This rating should be used for fishing vessels, where heavy continuous load is required and adverse operating conditions, such as poor maintenance and/or long periods of unattended running at high load factor, are involved. To cater for this requirement the fuel pump will in general be set to this rating and no overload allowance is provided.

The continuous rating can also be determined ac­cording to British Standard B.S. 649 or D IN 6270 A (Continent). According to these standards, an over­load of 10% above the continuous rating is obtainable for 1 hour in any period of 12 hours consecutive running. The fuel pump will be set to limit the output of the engine accordingly. I t will be clear that the user can operate at the 10% overload conditions con­tinuously. This means for the engine a continuous effective overload, which, for example, reduces the periods between overhauls.

In general the above mentioned ratings are for standard conditions of 750 mm Hg (29-5 inch Hg) at­mospheric pressure, intake air temperature up to 29°C (85°F) and a relative humidity of up to 60%. One has to decrease the output by 3-5% for every 300 m (1000 ft) over 150 m (500 ft) above sea level; 2% for every 6°C (10°F) above 29°C (85°F) ; 6% at a max­imum for the most adverse relative humidity.

When comparing the ratings of engines one has to be aware of the difference between the metric and British horse power. The former has a value of 75 kgf • m/sec, the latter 550 ft • lb/sec = 76-04 kgf • m/sec.

A P PE N D IX B

P O W E R D EFIN ITIO NS OF PR O PU L S IO N SYSTEMS

Description

brake power shaft power

delivered p ower th rust pow er effective power

SymbolI .T .T .C .

PbP s

Pd

P tPe

Form er Definition identification

BHPSH P

D H P

E H P

Ps — Pd + losses in propeller shaft P d = 2n - Q- nP t = T - V Ape = r t - v

In the above given definitions the following para­meters are used:

Q_, torque delivered at propeller n, rotational speed of propeller T, thrust of propellerVa , speed of advance of propeller = V (1 — w)R t , total resistance of ship (and gear)V, speed of vesselw, wake fraction (w = — 0-05 + 0-5 Cb)Cb , block coefficient